Modulation system for a microwave tube having depressed collector



Dec. 21, 1965 S. l. RAMBO MODULATION SYSTEM FOR A MICROWAVE TUBE HAVING DEPRESSED COLLECTOR Filed OOC. 8, 1963 I N VEN TOR.

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United States Patent O 3,225,314 MODULATION SYSTEM FOR A MICROWAVE TUBE HAVING DEPRESSED COLLECTOR Sheldon I. Rambo, Baltimore, Md., assignor, by mesne assignments, to the United States of America as represented bythe Secretary of the Navy Filed Oct. 8, 1963, Ser. No. 314,822 2 Claims. (Cl. 332-7) The present invention relates to a modulation system for a microwave tube and more particularly to a modulation system for a microwave tube such as traveling wave tube amplifiers and klystron amplifiers which are used in radar and communication transmitters.

It has been the practice to employ collector depression in traveling wave tubes and klystrons in order to provide improved eiciency. Electrons are rst emitted at the cathode, then accelerated by the anode or shell, and after interaction with the RF circuit, the electrons are slowed down by decelerating eld between anode and collector so as to be collected at a lower velocity. As a result, eiciency is increased, as less energy is Wasted in heating the collector.

Heretofore, two high voltage power supplies were provided for microwave tubes having depressed collectors. One power supply provided potential for the cathode and one provided potential for the collector. Also a pair of storage capacitors were provided to hold the top of the RF pulse up so as to prevent excessive droop. Two crowbar circuits were provided in order to remove the energy stored in the capacitors in case of a fault, so that the stored energy would not dump through the microwave tube arc and cause irreparable damage to the tube.

The present invention makes is possible to eliminate one power supply and one crowbar circuit and thus results in a safe, simple, modulating system. A linetype pulse modulator is provided and is used to pulse the collector, and as the line-type pulse modulator shares the cathode power supply and stores only enough energy for a single pulse, the components can be kept small. Also tube arcs between collector and ground are kept small and do not require a protective circuit.

It is therefore a general object of the present invention to provide an improved modulation system for a microwave tube wherein the collector is pulsed by a line-type modulator.

Still another object of the present invention is to provide an improved modulation system for a microwave tube that is simple and requires a minimum of parts.

Other objects and advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawing wherein:

FIGURE 1 is a diagram showing a preferred embodiment of the present invention;

FIGURE 2 is a chart showing the relationship of the various pulses produced by the embodiment of FIG- URE 1;

FIGURE 3 is a schematic view showing a line-type modulator; and

FIGURE 4 is a schematic view showing a typical crowbar circuit of the prior art.

In FIGURE 1 of the drawing, there is shown a microwave tube 11 such as a traveling wave tube amplifier that has an envelope 12, a cathode 13, a grid electrode 14, a collector electrode 15, and slow wave structure 16. Cathode 13 is connected to the negative terminal of a power supply 17 through lead 18, and collector electrode 15 is connected through transformer winding 19 and ice capacitor 21 to the positive terminal of power supply 17. A crowbar circuit 22 is shunted across capacitor 21 in order to protect microwave tube 11. Crowbar circuits are well-known in the art and employ a thyratron which, upon occurrence of a fault, ionizes and conducts damaging current away from the faulting tube. By way of example, a typical electronic-crowbar fault protection circuit is shown and described on pages 144-147 of the January 1956 issue of Electronics magazine and is illustrated in FIGURE 4 of the drawing. A fault in the protected tube 41 results in a sudden increase in current through resistor 42 producing a positive voltage pulse which is coupled by capacitor 43 to the grid of thyratron 44. This impulse ionizes thyratron 44 and causes it to conduct damaging current away from tube 41. Current through thyratron 44 energizes the vcoil of an overload relay 45 which causes circuit breaker 46 to open.

Referring now to FIGURES 1 and 3 of the drawing, a line-type modulator 23 is provided to pulse the collector electrode 15.

A charging impedance 24 and a charging diode 25 are connected in series between the power supply 17 and a pulse forming network 26. The charging impedance 24 limits the rate at which current is drawn from power supply 17 during the charging cycle and also acts as an isolation element during the discharge cycle and prevents the pulse-forming network 26 from discharging into power supply 17 instead of into collector 15. By way of eX- ample, the pulse-forming network 26 might be a lumpedconstant delay line consisting of an air-core inductance with taps along its length to which are attached capacitance to ground. The number of taps depends upon the pulse width and the fidelity required.

During the charging cycle, the charging inductance and the delay-line capacitance form a resonant circuit. 1f a D.C. voltage is suddenly applied, oscillations will occur. The peak voltage across the delay-line capacitance will be twice the supply voltage after the first half-cycle of oscillation. Thus the pulse repetition frequency (prf.) will be twice the resonant frequency. This method of operation, ignoring the effect of the charging diode, is often referred to as D.C. resonant charging. Charging diode 25, which is connected in series with the charging inductance, permits the modulator to be readily operated at any pulse repetition frequency, which is less than the prf. as determined by the resonant frequency. The function of charging diode 25 is to keep the delay line from discharging until the thyratron lires.

By way of example, the switching for a line-type modulator can be accomplished by a hydrogen thyratron 27. The advantage of a hydrogen-filled thyratron over an inert gas or mercury-filled thyratron is the rapid ionization and deionization time of hydrogen gas. The hydrogen thyratron also has better capacity for high-peak currents and can be designed to be relatively insensitive to temperature. The hydrogen thyratron 27 acts as a switch to discharge the stored energy through the primary 28 of a pulse transformer 29, the secondary 19 of which pulses the collector 15 of the traveling wave tube or klystron.

The pulses to the hydrogen thyratron 27 are provided by trigger generator 31 which also pulses the grid electrode 14 of the microwave tube 11. The pulses to grid electrode 14 are applied through delay line 32, pulse isolator 33, and modulator 34, as shown in FIGURE 1 of the drawing.

FIGURE 2 of the drawing shows the desired timing of the various pulses. The duration of the collector pulse (e4) is unimportant provided that the pulse (e4) begins before the grid pulse (e3) is applied and persists beyond the grid pulse duration.

In operation, the microwave tube 11 is energized by power supply 17, and in case of a fault in the tube, protection is provided by crowbar circuit 22. Trigger generator 31 provides a trigger pulse (el) to both the grid electrode 14 and modulator 23. Delay line 32 assures that the collector pulse (e4) begins before the grid pulse (e3) is applied. As shown in FIGURE 3 of the drawing, the trigger pulse (e1) is applied to the grid 'of thyratron 27 which, when turned on, stays on until the pulse-forming network 26 completely discharges itself, thus providing the rectangular pulse (e4).

It can thus be seen that the present invention provides an improved modulation system for microwave tubes that employ collector depression. Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood, that within the scope of the appended claims, the invention may be practiced `otherwise than as specically described.

What is claimed is:

1. A modulation system for a microwave tube comprislng;

a microwave tube including a cathode electrode, a grid electrode, and a collector electrode, a single power supply connected between said cathode electrode and said collector electrode, a storage capacitor connected between said cathode electrode and one terminal of said power supply, and pulsing means for providing to said grid electrode and said collector electrode, said collector electrode being connected to said pulsing means through a line-type Ipulse modulator. 2. A modulation system as set forth in claim 1 wherein a crowbar circuit is connected across said storage capacitor for protecting said microwave tube.

References Cited by the Examiner UNITED STATES PATENTS 3,046,492 7/1962 Dain et al 331-83 20 ROY LAKE, Primary Examiner.

ALFRED L. BRODY, Assistant Examiner. 

1. A MODULATION SYSTEM FOR A MICROWAVE TUBE COMPRISING; A MICROWAVE TUBE INCLUDING A CATHODE ELECTRODE, A GRID ELECTRODE, AND A COLLECTOR ELECTRODE, A SIGNAL POWER SUPPLY CONNECTED BETWEEN SAID CATHODE ELECTRODE AND SAID COLLECTOR ELECTRODE, A STORAGE CAPACITOR CONNECTED BETWEEN SAID CATHODE ELECTRODE AND ONE TERMINAL OF SAID POWER SUPPLY, AND PULSING MEANS FOR PROVIDING TO SAID GRID ELECTRODE AND
 2. A MODULATION SYSTEM AS SET FORTH IN CLAIM 1 WHEREIN A CROWBAR CIRCUIT IS CONNECTED ACROSS SAID STORAGE CAPACITOR FOR PROTECTING SAID MICROWAVE TUBE. 